drawing in latex (picture environment) + arrows (xy package)

I wanted to draw arrows in LaTeX to make that predictor-outcome-confounder diagram I saw in class. I found an answer here using the xy package in LaTeX.

I used to want to draw in LaTeX. pgf and tiks looks cool, but the learning curve looks too steep for me. I opted to R’s plotting and openoffice’s drawing capabilities. However, I just discovered the picture environment in LaTeX, which is not hard at all! It is also described here.